If removable battery/storage are important to you then the S3. Otherwise...

Frankly they are both very competent phones, and I think either will do you well. I'd honestly suggest the best you can do is go into a shop, have a play with both, and see whether you like one better than the other. If you do, then just go for that one.

Honestly there is very little separating the normal One X and the S3 so logic would say the X+ with its slight tweeks is either on par or slightly better, you would need a lab to detect a lot of the stuff though as they are both lightening quick.

If having a changeable battery and SD expansion is important to you then pick the S3, if you want the unibody design which is far better than the S3 pick the X+

I don't think the camera is better on the S3 as my missus has it and unless she is a worse photographer than me they look very similar, would say in bad light the S3 edges it though.

The UI is another bone of contention, I hate the look of touchwiz.. to me it wouldn't matter if you had gingerbread or jellybean on it, it would look the same. Sense is a lot more stripped back now and gives you the stuff you want (widgets) without bottlenecking the phone.

The screen is far better on the One X+, probably the best screen we can get in britain at the moment until we start getting the 1080 ones that are coming out.

Battery life is quite similar although my One X can go through the juice when playing tegra games but think that's to be expected, don't really bother with them now anyway. I think games look better on the One X but that could simply be the quality of the screen.

Honestly either phone you get will be great and it's a good forum for helping you work through the little quirks that both phones get.